[OpenMP][JIT] Fixed a couple of issues in the initial implementation of JIT

[tianshilei1992]

Summary

This patch fixes a couple of issues:

1. Instead of using llvm_unreachable for those base virtual functions, unknown value will be returned. The previous method could cause runtime error for those targets where the image is not compatible but JIT is not implemented.
2. Fixed the type in CMake that causes the Target CMake variable is undefined.

[tianshilei1992]

better failure message

openmp/libomptarget/plugins-nextgen/common/PluginInterface/CMakeLists.txt

 # NOTE: Don't try to build `PluginInterface` using `add_llvm_library` because we
 # don't want to export `PluginInterface` while `add_llvm_library` requires that.
 add_library(PluginInterface OBJECT
   PluginInterface.cpp GlobalHandler.cpp JIT.cpp)
 
 # Only enable JIT for those targets that LLVM can support.
 string(TOUPPER "${LLVM_TARGETS_TO_BUILD}" TargetsSupported)
 foreach(Target ${TargetsSupported})
-  target_compile_definitions(PluginInterface PRIVATE "LIBOMPTARGET_JIT_${TARGET}")
+  target_compile_definitions(PluginInterface PRIVATE "LIBOMPTARGET_JIT_${Target}")
 endforeach()
 
 # This is required when using LLVM libraries.
 llvm_update_compile_flags(PluginInterface)
 
 if (LLVM_LINK_LLVM_DYLIB)
   set(llvm_libs LLVM)
 else()

openmp/libomptarget/plugins-nextgen/common/PluginInterface/JIT.cpp

if (TT.isAMDGPU()) {
     LLVMInitializeAMDGPUTargetInfo();
     LLVMInitializeAMDGPUTarget();
     LLVMInitializeAMDGPUTargetMC();
     LLVMInitializeAMDGPUAsmPrinter();
     JITTargetInitialized = true;
   }
 #endif
   if (!JITTargetInitialized) {
-    FAILURE_MESSAGE("unsupported JIT target");
+    FAILURE_MESSAGE("unsupported JIT target: %s\n", TT.str().c_str());
     abort();
   }
 
   // Initialize passes
   PassRegistry &Registry = *PassRegistry::getPassRegistry();
   initializeCore(Registry);
   initializeScalarOpts(Registry);
   initializeVectorization(Registry);

openmp/libomptarget/plugins-nextgen/common/PluginInterface/PluginInterface.h

struct GenericDeviceTy : public DeviceAllocatorTy {
   }
   uint64_t getDefaultNumBlocks() const {
     return GridValues.GV_Default_Num_Teams;
   }
   uint32_t getDynamicMemorySize() const { return OMPX_SharedMemorySize; }
 
   /// Get target architecture.
   virtual std::string getArch() const {
-    llvm_unreachable("device doesn't support JIT");
+    return "unknown";
   }
 
   /// Post processing after jit backend. The ownership of \p MB will be taken.
   virtual Expected<std::unique_ptr<MemoryBuffer>>
   doJITPostProcessing(std::unique_ptr<MemoryBuffer> MB) const {
     return MB;
   }
 
struct GenericPluginTy {
   /// Get the number of active devices.
   int32_t getNumDevices() const { return NumDevices; }
 
   /// Get the ELF code to recognize the binary image of this plugin.
   virtual uint16_t getMagicElfBits() const = 0;
 
   /// Get the target triple of this plugin.
   virtual Triple::ArchType getTripleArch() const {
-    llvm_unreachable("target doesn't support jit");
+    return Triple::ArchType::UnknownArch;
   }
 
   /// Allocate a structure using the internal allocator.
   template <typename Ty> Ty *allocate() {
     return reinterpret_cast<Ty *>(Allocator.Allocate(sizeof(Ty), alignof(Ty)));
   }
 
   /// Get the reference to the global handler of this plugin.